A process in which a monomer is chemically polymerized using an oxidizing agent or a reducing agent and an electrolytic polymerization process in which a working electrode and a counter-electrode are placed in an electrolytic polymerization solution consisting of a combination of monomer/electrolyte/solvent and polymerization is conducted electrochemically by applying an electric voltage between the two electrodes are well known as polymerization methods for producing electrically conductive polymers such as polypyrrole, polythiophene, and others.
Since electrically conductive polymers having electrical conductivities currently used in practical applications are infusible and insoluble, the electrolytic polymerization process in which the desired polymer is deposited as a film on the working electrode is being adopted where the polymer is to be used in the form of a film. Further, since only specified substances react on the electrode surface in the process, in the electrolytic polymerization process inclusion of impurities is less apt to occur as compared with the chemical polymerization process.
In the electrolytic polymerization process, an electrically conductive polymer in film form is deposited on the working electrode surface and gradually grows into a thick film, upon application of an electric voltage between the two electrodes. In this case, the surface of the electrically conductive polymer film on its growing side tends to become rough. Further, in some cases, tubular structures form on the surface of the electrically conductive polymer film [F. T. A Vork & L. J. J. Janssen, Electro-chim Acta, 33 [11], 1513-1517 ('88)]. In extreme cases, the tubular structures grow into stalagmitic structures similar to "bellflowers" (refer to FIG. 1 attached hereto). Depending on the conditions, such "bellflowers" grow gregariously over the whole polymer film surface.
Although the mechanism of the formation of such "bellflowers" has not been elucidated, their development and growth are as follows. In the initial stage of the electrolytic polymerization, an electrically conductive polymer is deposited on the working electrode surface to thereby color the electrode. Development of "bellflowers" is not observed in this initial stage of polymerization, but small bubbles are generated on the electrode at a certain stage of the polymerization. Thereafter, nuclei parts of the "bellflowers" develop beneath the bubbles and, as the polymerization proceeds, the nuclei grow to lift up the bubbles, which also enlarge gradually. Although the "bellflowers" grow to various sizes depending on the polymerization conditions, there are cases where the diameter of the "bellflowers" becomes as large as 1 mm and the height as high as 2 to 3 mm when an electrically conductive polymer film having a thickness of about 10 to 200 micrometers is prepared.
The "bellflowers" can be easily removed from the film. However, not only the removal of the "bellflowers" causes the film surface to be rough thereby impairing the commercial value of the polymer film, but the resulting film has an uneven in weight distribution and there also is a cost disadvantage because the removed material has no commercial value.
In order to solve such problems, it has, for example, been proposed to cover the cathode with a porous diaphragm which is less gas-permeable; this expedient is based on the assumption that the cause of the development of surface roughness is the adhesion of bubbles formed at the counter-electrode to the polymer film being produced by polymerization (JP-A-1-230627). (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) On the other hand, it has now been found that lowering the current density (electric current value/area of immersed electrode) during electrolytic polymerization is effective. However, the former expedient is disadvantageous because the electrode structure becomes intricate, while the latter expedient has problems in that the rate of polymerization becomes low impairing production efficiency and in that the electrically conductive polymer film obtained has a reduced electrical conductivity.